Presence servers are increasingly being used to provide presence information such as the availability status of users. Presence information of a user identifies the current “presence state” of that user. Users may make their presence information available so that other users can decide how best to communicate with them. For example, the presence information may indicate whether a user is logged on (“online”) with an instant messaging server or is logged off (“offline”). Presence information may also provide more detailed information about the availability of the user. For example, even though a user is online, that user may be away from their computer in a meeting. In such a case, the presence state may indicate “online” and “in a meeting.”
In an instant messaging context, a publishing user (“publisher”) may provide their presence information to a presence service that then provides the presence information to subscribing users (“subscribers”). Thus, a presence service may use a subscriber/publisher model to provide the presence information for the publishing and subscribing users of the presence service. Whenever the presence information of a user changes, the presence service is notified of the change by that user's computer system and in turn notifies the subscribing users of the change. A subscribing user can then decide how best to contact the publishing user based on the presence information of the intended participants. For example, if the presence information indicates that a publishing user is currently in a conference telephone call, then the subscribing user may decide to send an instant message, rather than place a telephone call, to the publishing user. If the subscribing user, however, needs to call and speak with the publishing user, the subscribing user needs to monitor the presence information of the publishing user to know when the call can be placed. When the subscribing user notices that the publishing user's presence information indicates that the telephone conference has been concluded, the subscribing user can then place the telephone call.
Because of the increasing popularity of instant messaging systems and other real-time communications systems, presence services need to support an increasing numbers of users. In addition, these systems need to support increasingly complex definitions of “presence information.” For example, presence information may include calendar information that specifies the availability of a publisher during the next several months. As a result, a presence server is typically developed to provide presence services in an efficient way. Unfortunately, typical presence models require intense computation when presence information is updated. Thus, as more publishers and subscribers are added, additional presence servers are needed to perform the necessary computations.
As one example, the Internet Engineering Task Force (“IETF”) has published a proposed draft presence model entitled “A Data Model for Presence.” That presence model describes that the presence information of a publisher is published by that publisher in an XML document with an associated XML schema. A publisher can update their presence information of the XML document using edit commands such as pointing to a line in the document and replacing it with the word or pointing to a node within an XML subtree and replacing it with another XML subtree. This presence model requires the presence server to parse the XML document based on the XML schema to locate the presence information within the document that is to be updated. The parsing of the presence document can be computationally expensive and may need to be performed on every update. This presence model also allows a subscriber to specify filters for identifying under what conditions the subscriber is to be notified of changes to presence information. For example, a subscriber may request to be notified only when a presence status changes from out-of-office to in-office. Whenever presence information changes, however, the presence server needs to reapply the filter for each subscriber to determine whether the subscriber is to be notified of the updated presence information. The reapplication of the filters by a presence server can also be computationally expensive.